1. 1 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Other Molding Processes While extrusion and injection molding processes are by far the most common types of methods used for molding polymers, other molding methods can offer better economic, quality and production advantages depending on the application In many cases (such as for very large items like pressure tanks, furniture and appliance casings) injection nor extrusion processes can feasibly handle the job However, extrusion or injection molding may provide a preform shape that other methods then form Thermoforming of polymer sheet uses extruded, skived or molded sheet stock Thermoplastic welding uses rod stock that has been extruded

2. 2 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Casting Casting is the process by which plastic is placed in a mold and allowed to harden; relying on gravity, atmospheric pressure and/or centripetal forces to form the material into the mold There are several forms of molding which are considered castings Simple casting Film casting Slush and Static casting Rotational casting (Rotational molding)

3. 3 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Simple Casting True to its name, simple casting is one of the simplest methods of forming a plastic; it involves simply pouring a molten thermoplastic, liquid thermoset resin or monomer and catalyst mixture into an open mold to harden Many products are made by simple casting Sheet for thermoforming optically clear parts is often cast Polyesters are cast to make decorative items such as picture frames and statuettes, or mixed with fillers to make items like fabricated marble tops and fabricated wood Silicones and epoxies are often cast for the purpose of producing molds or encapsulating electronics or other components

4. 4 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Acrylic Sheet Casting Cast sheets of acrylic have superior optical clarity over extruded sheet in addition to having less residual strain and no preferred directionality of the polymer chains Sheet can be produced continuously or in a batch process called Cell Casting Continuous casting uses polished stainless steel belts to form the sheet, cell casting will typically employ polished and or tempered glass to form the sheet (better clarity than using steel) In either process, a catalyzed resin (methyl methacrylate) is poured between the molding sheets and allowed to polymerize under controlled heating The sheet is allowed to cool in the mold before removing it About 90 million lb./year of continuous cast acrylic sheet and 10 million lb./year of cell cast sheet are used in the US (Modern plastics Encyclopedia Handbook) Nylon can also be cast in similar manner

5. 5 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Cell Casting

6. 6 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Film Casting Film casting typically uses polymers that can be put in solution, but may also use un-polymerized resins In film casting a thin sheet of solution or low viscosity resin is applied to a heated roller or wheel Solvent is evaporated away or the film is polymerized leaving the consolidated polymer to be taken up on rollers

7. 7 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Film Casting

8. 8 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Slush and Static Casting Slush and static casting use equivalent molding techniques but the material molded is different Slush casting involves the use of liquid casting material (like organosol and plastisol) Plastisol is primarily a suspension of PVC particles and plasticizers; organosol contains a higher percentage of PVC in a solvent suspension and may or may not contain plasticizers PVC particles are typically produced by emulsion polymerization Static casting involves the use of polymer powders The molding process involves pouring the liquid or powdered resin into a heated mold; the emulsion particles or solid powders conform and stick to the wall while building up thickness as particles coalesce and melt together on the hot mold In the case of oganosols and plastisols, solvents are evaporated and excess plasticizer is volatilized When the desired thickness is obtained, excess resin is poured from the mold, leaving a wall covering of semi-consolidated polymer A final heating is required to fully coalesce the part and drive of any excess solvent (organosol) and fully absorb the plasticizer (plastisol)

9. 9 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Basic Slush or Static Casting Text Pg 229

10. 10 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Rotational Casting (Molding) Rotational molding, like slush and static casting, is used to make seamless hollow parts; however, unlike static or slush casting the mold is closed and rotated to attain a more even and controlled wall thickness It has typically been used to make very large tubes, tanks storage bins and other large hollow items, but almost any hollow part can be made in this manner Rotational molding is accomplished on a two axis rotation which ensures even distribution of material along the wall Both thermoplastic powders and liquid polymers can be processed but typically the process has been used for thermoplastics and crosslinked polyethylene Initially, powder is charged into the mold and sealed; the mold is then heated and a porous layer of powder forms on the surface; as the polymer is melted at the mold wall, particles fuse together and form an impermeable layer of material

11. 11 4/28/2003 Tech 140 Lecture – Casting & Rotational Molding Schematic of Rotational Molding Unit Text pg231